1. Field of the Invention
The present invention concerns a gravity casting method.
2. Description of the Related Art
Parts, for example, knuckles used for automobiles are generally cast by a gravity casting method using cast iron, aluminum, or aluminum alloys as a base material. The gravity casting method is a method of filling a molten metal in the entire cavity by utilizing the weight (gravitational force) of the molten metal filled in a feeder upon pouring the molten metal from a runner of a molding die into a cavity and casting the same therein.
As existent gravity casting methods, a stationary casting method of conducting casting while keeping a molding die in a horizontal state, a turnover casting method of pouring a molten metal and then solidifying the same while keeping a molding die in a state rotated by a predetermined angle, and a casting method of tilting or vibrating a molding die have been known (refer to JP-A-2005-193262). However, when it is intended to conduct the step of cooling and solidifying the molten metal poured into the cavity or the like while vibrating the molding die under tilting as described in JP-A-2005-193262, since the molding die is not in the horizontal state, it may result in a problem that the effect of prevailing the molten metal to all top end portions in the cavity under the gravitational effect by the molten metal in the feeder portion (feeder melt) is particularly difficult to be obtained. In addition, in a case where a vibration mechanism has to be provided to the casting apparatus, the device cost increases inevitably.
At first, a gravity casting method according to an existent embodiment is to be described. For example, in a stationary casting method shown in FIG. 7, a product is cast by holding a molding die 101 in a horizontal state, filling a melt (molten metal) poured from a runner 112 so as to prevail to the top end in a cavity 114 by the gravitational force of the molten metal filled in a feeder portion 116 and then cooling to solidify the filled molten metal. In this case, a metal lump formed by cooling the molten metal filled in the flow channel or the like of the runner 112 (hereinafter referred to as “non-product part”) is deposited to the cast product.
Since such a non-product portion is a portion which has to be cut off from the cast product and the yield is worsened as the non-product is steeper, it is required to be as small as possible.
For decreasing the non-product portion thereby improving the yield in the existent stationary casting method, the following two methods may be considered. The first is a method of simply shortening the length of a runner 112 to make the non-product portion smaller as shown in FIG. 8. However, as apparent from the drawing, when the length of the runner 112 is merely shortened, since the height of a sprue 110 becomes lower than the height at the upper end of feeder portion 116 and the molten metal is not filled as far as the feeder portion 116, this may cause a problem that the amount of the feeder melt necessary for prevailing the molten metal as far as the top end of a cavity 114 cannot be ensured. The second is a method of shortening the length of a runner 112 while maintaining the height of a sprue 110 necessary to fill the molten metal as far as the upper end of the feeder portion 116 by making the inclination angle γ of the runner 112 steeper than that of a usual angle β (γ>β) as shown in FIG. 9. According to the method, the amount of the feeder melt necessary for filling the molten metal as far as the upper end of the feeder portion 116 can be ensured. However, as the inclination angle of the runner 112 becomes steeper, a turbulent flow occurs upon pouring of the molten metal into the cavity 114 thereby resulting in a problem that blow or oxide is generated in the product portion (particularly, a portion surrounded by a dotted circle in the drawing).
On the other hand, in a turnover casting method shown in FIG. 10, a runner 112 is provided between a feeder portion 116 and a cavity 114 and a molding die 101 is rotated by a predetermined angle (90° in this case) in a stage where a molten metal 103 is filled in a feeder portion 116 and the cavity 114, and the product is cast by cooling and solidifying the filled molten metal while maintaining the die at that state. In this case, by the provision of the runner 112 between the feeder portion 116 and the cavity 114, the size of the non-product portion can be decreased depending on the simple shape of the product. However, in a case where the molding product has such a complicated shape that arms extend radially, for example, as that of a knuckle used for automobiles, there may be a problem that the feeder portion should be provided on every arm and, as a result, the yield is worsened.
As described above, upon casting a product, particularly, a knuckle used for automobiles, etc. in which the shape of the molding product is formed while being extended radially in the gravity casting method, the molten metal has to be prevailed as far as the top end thereof. However, in the case of the stationary casting method, it was impossible to ensure the amount of the feeder melt capable of prevailing the molten metal as far as the top end of the molding die unless the size of the feeder portion is made unnecessarily larger or the length of the runner is unnecessarily greater. Accordingly, there was a problem that the yield was poor. In this case, when the runner is shortened and the angle is made steeper, although the yield is improved, a turbulent flow is formed in the molten metal thereby possibly generating blow or oxide in the product portion.
On the other hand, in the case of the turnover casting method, there was a problem that the yield is poor, for example, that the feeder portion has to be provided at plurality of portions depending on the shape of the molding product.
Further, in the case of the casting method of pouring the molten metal while situating the molding die gradually horizontally from the tilted state, there was a problem that the molten metal is difficult to be prevailed as far as the top end of the molding die depending on the shape of the molding product.